1. Field of the Invention
The invention relates to a molding method and apparatus for a vehicular lamp part. More specifically, the invention relates to a molding method and apparatus for molding a vehicular lamp part having an undercut portion without making a die complex.
2. Description of Related Art
In a vehicular lamp part, there are cases in which forming an undercut portion is unavoidable.
For example, when a light source bulb 3 is attached to the center of a rear portion of a lamp body 2, as in a vehicular lamp 1 shown in FIG. 5, a waterproofing rib 4 protrudes toward the rear so as to surround a portion of the light source bulb 3 (or a socket holding the light source bulb 3) that protrudes from the rear surface of a lamp body 2. The rear end of this waterproofing rib 4 is abutted against a vehicle body (not shown) via a seal member 5, to make the area around the light source bulb 3 watertight.
When this type of waterproofing rib 4 is formed, the protruding length is quite large. As a result, if the waterproofing rib 4 is formed with an equal thickness from the front end (the end adjacent to the lamp body 2) to the rear end, it is not possible to release it from the die during injection molding. Therefore, the waterproofing rib 4 is formed with tapered surfaces 4a, 4a, referred to as so-called draft angles (see FIG. 6). When these kinds of tapered surfaces 4a, 4a are formed and a predetermined thickness is maintained at the rear end for strength, the thickness of a base portion 4b inevitably correspondingly increases. An increase in the thickness of the base portion 4b means an increase in mass of material resin around the base portion 4b, which results in a large difference in cooling speed, after injection, between the surface and the inner portion. This difference causes a depression 6, referred to as a so-called sinking, and therefore compromises the appearance. With a lamp body of a vehicular lamp, reflective surface treatment by vapor deposition or painting or the like is often applied to the inside surface, so even a slight deformation of the surface is highly visible.
Therefore, as shown in FIG. 7, a tapered surface 4′a is formed on one side of a waterproofing rib 4′ and an undercut portion 4′c is formed on the other side. Undercut portion 4′c has a slanted surface which is slanted with respect to a die closing/opening direction, i.e., the moving direction of a movable die when the die is closed and a cavity is formed, and the moving direction of the movable die when the die is opened and the cavity is opened. Thus, the thickness of the base portion 4′a of the waterproofing rib 4′ is not made too thick and, as a result, the sinking 6 described above can be inhibited from occurring.
However, when a molded part has an undercut portion, the undercut portion is not able to be released from the die by simply moving the die in the die opening direction. Therefore, a portion of the movable die, i.e., a die surface which corresponds to the undercut portion, is constructed of a sliding core that moves in a direction that intersects with the die closing/opening direction with respect to the movable die. That is, after injection molding, the sliding core is first moved and separated from the undercut portion, and then the die is opened.
Providing the sliding core, however, is problematic in that it leads to an increase in the cost of the die due to the complexity of the movable die. Thus, methods have been developed to enable a molded part that has an undercut portion to be released from a die without using a sliding core so that the cost of the die does not increase remarkably, such as shown in: (1) Japanese Utility Model Publication Sho. 57-6515; (2) Japanese Utility Model Publication Sho. 59-29216; (3) Japanese Utility Model Publication Hei. 5-58249; and (4) Japanese Patent Application Laid-Open Publication No. Hei. 9-155930.
However, these methods still fail to provide a die construction and operation that are sufficiently simple and have sufficient cost reduction.
Therefore, in view of the foregoing problems, it is an object of the invention to mold a vehicular lamp part having an undercut portion without making the die complex.